1. Field of the Invention
The present application relates to a method for forming carbon/carbon composites suited for use as friction-bearing and structural materials for high temperature applications. It finds particular application in conjunction with a composite material formed by resistance heating of carbon fiber/binder mixtures during application of a compressive force and will be described with particular reference thereto. It should be appreciated that the method has application in other areas where the combined effects of pressure and temperature are desired.
2. Discussion of the Art
Carbon/carbon composites include those structures formed from a fiber reinforcement, which itself consists primarily of carbon, and a carbon matrix derived from a thermoplastic binder, such as pitch, or a thermosetting resin, such as a phenolic resin. Such materials are useful in applications where high temperature frictional properties and high strength to weight ratio are important. For example, carbon/carbon composites are known to be effective for providing thermal barriers and friction-bearing components, particularly in aircraft, aerospace vehicles, and high performance road vehicles. Carbon/carbon composites have been used for forming brake pads, rotors, clutches, and structural components for these vehicles. They tend to exhibit good temperature stability (often up to about 3000xc2x0 C., or higher), high temperature friction properties (typical coefficients of friction are in the range of 0.4-0.5 above 500-600xc2x0 C.), high resistance to thermal shock, due in part to their low thermal expansion behavior, and lightness of weight. Thermal insulation materials formed from certain types of carbon fibers exhibit excellent resistance to heat flow, even at high temperatures.
A common method of forming carbon/carbon composites begins with layup of a woven fiber fabric or pressing a mixture of carbonized fibers, such as cotton, polyacrylonitrile, or rayon fibers, and a fusible binder, such as a phenolic resin or furan resin. In the process, the fibers are first impregnated with resin to form what is commonly known as a prepreg. Multiple layers of the prepreg are assembled in a mold of a heated press. The prepreg is compressed while simultaneously applying heat to the mold at temperatures of 200xc2x0 C.-350xc2x0 C. for a period of six hours or more to cure the resin fully. The fiber and cured resin composite is then heated at a slow rate to a final temperature of about 800xc2x0 C. in a separate operation to convert the binder to carbon. This carbonization step is carried out in an inert atmosphere and often takes about eighty hours to complete. Typically, the density of the carbon composite thus formed is up to about 0.6 to 1.3 g/cm3.
For applications such as brake components and other friction-bearing applications, a density of about 1.7 g/cm3 or higher is generally desired. To reduce voids in the pressure and heat-treated preform and increase its density, the preform is infiltrated with a phenolic resin or other carbonizable matrix material using a vacuum followed by pressure and the infiltrated material is then carbonized by heating. Densification is also often accomplished by chemical vapor infiltration (CVI) or chemical vapor deposition (CVD). The selected infiltration process is generally repeated six to ten times before the desired density is achieved. A final processing step may include graphitization of the preform by heating it in an inert atmosphere to a final temperature not exceeding about 3200xc2x0 C. Above this temperature, carbon from the composite material tends to vaporize.
The lengthy heating and infiltration times render such composites expensive and impractical for many applications. For example, it may take about five months to form a carbon/carbon composite article, depending on the number of densification steps. Accordingly, sintered metal articles are commonly used for thermal applications, despite their greater weight and often poorer thermal stability and friction properties.
The present invention provides a new and improved method of forming a dense carbon/carbon composite, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of forming a composite material is provided. The method includes combining a reinforcement material, which includes carbon-containing fibers, with a carbonizable matrix material to form a mixture and heating the mixture to a sufficient temperature to melt at least a portion of the matrix material and remove at least a portion of the volatile components from the matrix material. The heating step includes applying an electric current to the mixture such that heat is generated within the mixture. While heating the mixture, a pressure of at least 35 Kg/cm2 is applied to the mixture to form a compressed composite material.
In accordance with another aspect of the present invention, an apparatus for forming a compressed composite material is provided. The apparatus includes a vessel, which defines a cavity for receiving a material to be treated. A means for applying pressure applies a pressure of at least 35 kg/cm2 to the material in the cavity. A source of electrical current applies a current through the material to resistively heat the material. A temperature detector detects the temperature of the material. A control system controls the pressure applying means and the source of electrical current such that the mixture is sequentially heated at a first temperature and pressed at a first pressure for a first period of time, and heated at a second temperature higher than the first temperature and pressed at a second pressure higher than the first pressure for a second period of time.
In accordance with another aspect of the present invention, a method of forming a composite material suitable for vehicle brakes is provided. The method includes compressing a mixture of carbon fibers and a matrix material which includes pitch. During the step of compressing, a current is applied to the mixture. The mixture provides sufficient electrical resistance to the current such that the mixture reaches a temperature of at least 500xc2x0 C. to drive off volatile components of the mixture and form a compressed preform. A carbonizable material is introduced into voids in the compressed preform to form an impregnated preform. The product may be heated to carbonize the carbonizable material. The introduction and baking steps are optionally repeated. The impregnated preform is graphitized to a final temperature of from about 1500xc2x0 C. to about 3200xc2x0 C. to form the composite material. The composite material has a density of at least 1.7 g/cm within two introduction steps.
An advantage of at least one embodiment of the present invention is that carbon-carbon composites, such as insulation and brake component materials, are formed in much shorter periods of time than by conventional hot pressing methods.
Another advantage of at least one embodiment of the present invention is that the density of the hot pressed material is higher than in conventional preforms, thereby enabling desired densities to be achieved with fewer densification and carbonization cycles.
Another advantage of at least one embodiment of the present invention is that a composite material is formed using fewer processing steps.